Concrete mix is comprised of mainly Portland cement, aggregates (gravel, sand), water and admixtures. A chemical reaction between cement and water forms hydrated cement paste (referred to as cement gel) that when hardened binds the aggregates together to give concrete its characteristic strength.
The porosity of hydrated cement gel consists of two types of pores: gel pores and capillary pores. For water-to-cement weight ratios at or below 36%, the porosity of cement gel is due to the porosity of hydrated cement gel. At water-to-cement weight ratios above 36%, the porosity of hydrated/hardened cement gel becomes partly due to the cement gel, and partly space not filled by the cement gel; that which is left behind by unused water upon evaporation, called “capillary pores”. The higher the initial water-to-cement weight ratio above 36%, the greater the capillary pores volume.
The cement gel pores are very small (0.0005-0.01 micrometers). Water in such small pores, below 0.0033 micrometers, cannot freeze down to temperatures as low as −40° C. On the other hand, since capillary pores are much larger (0.02-10 micrometers), the water in these capillary pores freezes quite easily. Water freezing in the capillary pores expands and if the expansion is not adequately relieved, it can cause excessive stress to the walls of cement gel pores and cause the brittle cement gel pore walls to break (typical tensile strength of cement gel is about 1000 psi), causing deterioration of the concrete over repeated freeze-thaw cycles.
Widely practiced water-to-cement weight ratios in wet concrete mix are 40-60% (generally 40-50%) for good workability of wet concrete mix, but when the water-to-cement weight ratio is greater than 36%, the subsequently hardened concrete exhibits poor durability under freeze-thaw cycles. In order to ameliorate this problem, the conventional practice is to entrain about 4-8% by volume air in the wet concrete mix, for making the resulting hardened concrete more durable under freeze-thaw cycles.
Entrained air in a wet concrete mix is in the form of well-dispersed air bubbles and is achieved by adding special chemical admixtures, known as Air Entraining Agents (AEA), in the wet concrete mix. AEAs are generally comprised of wetting agents, surfactants, and/or foaming agents. The entrained air bubbles provide space for accommodating expansion of water freezing in the capillary pores, thus preventing the walls of the cement gel pores from experiencing excessive stress that could cause cement gel walls to crack. An entrained air content of about 4-8% by volume in the conventional concrete matrix corresponds to about 16-32% by volume in the cement gel (assuming the most common value of about 25% cement gel volume in concrete matrix). This is a huge volume fraction of a non-strength-contributing factor in the concrete matrix (cement gel).
Entrained air bubbles produced by AEAs can have a wide dimension distribution and maximum air bubble dimension, such as from 10-1000 micrometers (e.g., 90% of air bubbles of dimension above 300 micrometers, as reported in “Investigation into Freezing-thawing Durability of Low Permeability Concrete with or without Air Entraining Agent”, June 2009, National Pavement Concrete Center, Iowa State University, Ames, Iowa). The entrained air bubbles accommodate expansion of water freezing in capillary pores. Typically, the tensile strength of a cement gel is about 1000 psi, but water freezing in capillary pores can travel up to 550 micrometers under 1000 psi of pressure generated by the expansion of the freezing water.
Accordingly, the American Concrete Institute recommends spacing between air bubbles (also termed the Spacing Factor) of no more than 200 micrometers for entrained air bubbles to be effective in making concrete freeze-thaw durable. In addition, because of the very large dimension distribution of entrained air bubbles, including air bubble dimensions as high as 1000 micrometers, the specific surface area of entrained air bubbles is recommended to be greater than 20 mm2/mm3, in combination with a Spacing Factor of 200 micrometers, as recommended by ACI (specific surface area is calculated as total surface area of entrained air bubbles divided by total volume of entrained air bubbles).
Although these conventional freeze-thaw durable concrete mixes provide good freeze-thaw durable concrete, the entrained air negatively impacts the compressive strength of concrete in a significant manner, reducing compressive strength of the concrete by about 5% for every 1% increase in entrained air (reference: US Army Corps of Engineers Report No. ERDC/CRREL TR-02-5, February 2002). The loss in compressive strength of concrete due to entrained air is of significant concern in case of high strength concrete. High strength concrete is often made with silica-fumes and water-to-cement weight ratios below 40%. Such concrete without entrained air has shown very high initial compressive strength, but exhibits low durability under freeze-thaw cycles for water-to-cement weight ratios as low as 36%. High strength concrete using silica fumes and water-to-cement weight ratios of about 25% can exhibit very high strength as well as high freeze-thaw durability. However, it is virtually impossible to make such low water content concrete mixes consistently, as outlined in the above referenced US Army Corps of Engineers Report No. ERDC/CRREL TR-02-5. There are many factors that are difficult to control affecting air bubble dimension distribution and average bubble dimension, for example the nature of the admixtures used, their compatibility with other ingredients in the concrete mix, the types of cement and aggregates, water quality parameters like hardness, environmental conditions, etc., which makes air entrained concrete noticeably variable in performance-in-place.
The presence of large air bubbles and a large volume fraction of air in the cement gel have a pronounced negative synergistic effect on strength properties of such concrete. For example, US Army Corps of Engineers Report No. ERDC/CRREL TR-02-5 reports a 5% decrease in compressive strength of concrete for each 1% increase in entrained air content. Said synergistic effect can also reduce other strength-related properties of hardened concrete, like abrasion resistance, toughness, impact strength, and thus, reduce the overall durability of concrete in use; e.g., infrastructures, highways.
It is also known in the art to make light weight concretes (LWC), which are comprised of hollow spheres and formulated to have densities generally below 1.5 g/cm3, in contrast to most widely used concretes having densities above 2 g/cm3. In these materials, low density is achieved by using high volume fractions of low density fillers like hollow spherical elements, generally at least about 25 vol. % (irrespective of whether light weight hollow elements/fillers are organic, inorganic, polymeric, metallic, hybrid or combinations thereof).
Although volume fraction is the most important parameter affecting the density of LWC, often the amount of hollow elements/fillers in LWC is reported in weight percentage. The weight fraction of hollow elements can be calculated as follows:Weight fraction=(volume fraction×true density of hollow element)/(density of LWC)The weight fraction of hollow elements/fillers in LWC for reducing density of concrete is generally at least 2%.
Accordingly, there is a need for providing a concrete mix formulation that can produce wet concrete mixes having good workability, as compared to the workability of conventional freeze-thaw durable wet concrete mixes having 4-8% entrained air, and a resulting hardened concrete having improved compressive strength and good freeze-thaw durability, again as compared to the compressive strength and the freeze-thaw durability of hardened concrete obtained from conventional freeze-thaw durable wet concrete mixes having 4-8% entrained air.